Clay Induced Changes in the Aggregation Pattern of Safranine-O In

Home , Phenazine, Safranin

Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:

Chemistry

journal homepage: www.elsevier.com/locate/jphotochem

Invited feature article

Clay induced changes in the aggregation pattern of Safranine-O in

hybrid Langmuir-Blogdgett (LB) ﬁlms

Mitu Saha, Ashis Shil, S.A. Hussain, D. Bhattacharjee*

Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar, 799022, Tripura, India

A R T I C L E I N F O A B S T R A C T

Article history:

Received 7 June 2017 In the present communication, detailed investigation has been carried out to study the clay mineral

Received in revised form 16 July 2017 induced changes in aggregation of a well known cationic ﬂuorescent dye Safranine-O (SO) in aqueous

Accepted 19 August 2017 solution as well as in organo-clay hybrid Langmuir-Blodgett (LB) ﬁlms. At higher dye concentration in

Available online 24 August 2017

aqueous solution, Safranine-O (SO) formed both H-dimers and J-aggregates and the corresponding UV–

vis absorption spectra got distorted with increasing solution concentration. In clay mineral Laponite

Keywords:

dispersed aqueous solution SO formed J-aggregates with increasing Laponite concentration resulting in

Surfaces

an increase in ﬂuorescence intensity. H-dimeric sites were also sufﬁciently decreased in organo-clay

Thin ﬁlms

hybrid LB ﬁlms of SO. UV–vis absorption and ﬂuorescence spectroscopic as well as in-situ Brewster Angle

Multilayers

Microscopic (BAM) studies were employed in our investigation.

Optical properties

Surface properties

Phase transitions

1. Introduction in the aqueous solution, due to the aggregation of the dye

molecules into dimers and higher order aggregates [11]. Safranine-

The well-known cationic ﬂuorescent dye phenazine dye O molecules form J- and H-aggregates due to the interactions of the

Safranin-O (3,7-diamino-2,8-dimethyl-5-phenyl phenazinium hydrophobic n-p stacking and the electrostatic interactions

chloride) abbreviated as SO, consists of a phenazinium nucleus between the anionic and cationic groups under various conditions

in the molecular ring system [1] (inset of Fig. 1(A)). This dye has [1].

remarkable sensitivity to the surrounding medium, it has been SO is water soluble because it is an ionic compound. It was

extensively used in many research areas as photosensitizers in reported in several works that water soluble ionic molecules could

electron and electron-transfer reaction [2–5], as a sensitizer in be electrostatically adsorbed onto a oppositely charged preformed

visible light photopolymerization [6–8], in the textile, pharmaceu- Langmuir monolayer and thus forming a complex Langmuir

tical, paper, cosmetic industries [9,10], as probes for studying monolayer at the air-water interface of the Langmuir Trough

various micro heterogeneous environments including micelles, [22–25]. SO being cationic can be adsorbed from the aqueous

reverse micelles and polymeric matrices [11–14] and in many solution of the Langmuir Trough to a preformed anionic Langmuir

biological applications in photochemistry [15,16], DNA determina- monolayer and consequently forms a complex Langmuir mono-

tion [17] etc. In biological labeling as ﬂuorescent probe, various layer. In some case inorganic clay minerals can be incorporated

safaranine derivatives have been used [18]. J-aggregating proper- onto the Langmuir monolayer thus forming a hybrid ﬁlm. A

ties of SO has been used for drugs targeted to DNA and also for the complex/hybrid Langmuir monolayer thus formed at the air-water

labeling of the DNA [19]. Safranine dyes showed metachromasy interface can be transferred onto solid substrate to form mono- and

when interacted with anionic polyelectrolytes and has vast multilayered Langmuir-Blodgett (LB) ﬁlms. Depending on the

applications in the field of textile effluent treatment and also in various film forming parameters and molecular organizations in

the staining of biological tissues [20,21]. the LB ﬁlms various kinds of dye aggregates become possible. In the

In dilute aqueous solution of SO, UV–vis absorption spectrum UV–vis absorption spectrum, aggregated species cause large

shows intense monomeric band with peak at 520 nm [11,19]. The spectral shifts with respect to the monomer.

absorption band gets distorted with increasing dye concentration Red shifted absorption band (Bathochromic shift) with respect

to the monomer absorption band is referred to as J-band formed

due to J-type molecular aggregates. H-aggregates resulted in the

blue shifted absorption band (Hypsochromic shift) with respect to

* Corresponding author.

the monomer and it generally leads to the formation of H-dimer

E-mail address: [email protected] (D. Bhattacharjee).

http://dx.doi.org/10.1016/j.jphotochem.2017.08.053

200 M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208

Fig. 1. [A] p-A isotherms of (I) pure SA monolayer, (II) SA-SO complex monolayer; [B] C-p graphs of (I) pure SA monolayer and (II) SA-SO complex monolayer.

resulting in the quenching of ﬂuorescence intensity [26–28]. J- 2.2. Instruments

aggregates are formed by end-to-end staking and H-aggregates are

formed by face-to-face staking of the dye molecules [29–31]. A commercially available Langmuir-Blodgett (LB) ﬁlm deposi-

According to Kasha et al. [32] different aggregated species are tion instrument (Apex 2000C, Apex Instruments Co., India) was

formed due to strong intermolecular interactions between used for surface pressure vs. area per molecule (p-A) isotherm

monomeric species and delocalized excitonic energy over the measurements of complex/hybrid Langmuir monolayers and also

whole assembly of the aggregates. H-aggregate is non ﬂuorescent preparation of mono- and multilayered LB Films. The Brewster

in nature and reduces the ﬂuorescence intensity of the dye Angle Microscope (BAM) images of complex/hybrid ﬁlms were

molecule. H-aggregation of Safranine-O has been used as photo- taken by a commercially available in-situ BAM (Accurion, nano-

sensitizers in semiconductors and also initiation of photopolyme- ﬁlm_EP4) attached to a KSV NIMA Langmuir-Blodgett instrument.

rization [33]. On the other hand, J-aggregate is highly ﬂuorescent Ultra-pure Milli-Q (18.2 MV-cm) water was used for the prepara-

and J- aggregation of SO has vast applications in optical storage, tion of the aqueous subphase and Laponite dispersed subphase of

ultrafast optical switching, spectral sensitization, light emitting the Langmuir trough and also for preparation of aqueous solution

diodes, lasers and Q-switches and also used as a ﬂuorescent probe of SO. Aqueous Laponite dispersed subphase was used at various

for biological labeling etc. [34–39]. clay concentrations ranging from 10 À 80 PPM. Aqueous Laponite

In the present work, detailed investigations have been carried dispersion was stirred for 24 h and then sonicated for 30 min prior

out to study the effect of clay mineral Laponite, on the aggregation to use. The temperature was maintained at 24 C throughout the

behavior of Safranine-O in solution as well as in ultra thin ﬁlms. In experiment. UV–vis absorption and ﬂuorescence spectra were

recent time, inorganic nano-clay minerals have shown great recorded by UV–vis absorption spectrophotometer (Lambda 25,

promise for the construction of hybrid organic/inorganic nano- Perkin Elmer) and Fluorescence spectrophotometer (LS-55, Perkin-

materials due to their unique material properties, colloidal size, Elmer) respectively. For spectroscopic characterizations, LB ﬁlms

layered structure and nano-scale platelet shaped dimension. These were prepared on thoroughly cleaned quartz substrates. The quartz

organo-clay hybrid ﬁlms have various technological applications substrates were cleaned with soap solution for removal of grease/

due to their unique semiconducting, conducting, nonlinear, dirt. Then the substrates were treated with chromic acid for 30 min

dielectric properties [40,41]. and washed in de-ionized water. Further they were cleaned with

acetone and stored in a dry oven.

2. Experimental section

2.3. Methods

2.1. Chemicals

Stock solutions of SA (0.5 mg/ml) and ODA (1 mg/ml) were

Safranine O (SO), Stearic Acid (SA) and Octadecylamine (ODA) prepared using spectroscopic grade chloroform. Stock solution of

purity >99% were purchased from Sigma-Aldrich Chemical SO was prepared by dissolving it into ultra-pure Milli-Q water

À4

Company and used as received. Working solutions were prepared (1.0 Â 10 M). In order to measure the p-A isotherm of pure SA

by dissolving them in spectroscopic grade chloroform (SRL) and its monolayer, 60 ml of chloroform solution of SA was spread on the

purity was checked by ﬂuorescence spectroscopy before use. The aqueous subphase of the Langmuir Trough by using a micro-

clay mineral Laponite used in this study was obtained from the syringe. After complete evaporation of the volatile solvent, the

source clays repository of the clay minerals society. Cation barrier of the Langmuir Trough was compressed slowly to record

cxchange capacity (CEC) of Laponite is 0.74 meq/gm. the isotherm. In case of SA-SO complex monolayer, 8000 ml SO

M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208 201

À5

(5.0 Â 10 M) solution was mixed in the aqueous subphase and sufﬁcient time when some domain structures were observed at

then 60 ml of SA solution was spread on the mixed aqueous the air-water interface then the barrier of the Langmuir Trough was

subphase. After waiting sufﬁcient time to complete the reaction, slowly compressed and BAM images were taken.

monolayer was compressed slowly to obtain p-A isotherm. Here

cationic SO molecules interacted electrostatically with the anionic 3. Results and discussions

SA molecules and formed SA-SO complex Langmuir monolayer at

the air-water interface. The complex monolayer was also 3.1. Molecular structures of SA and SO and formation of complex

transferred onto solid substrates at different surface pressures molecules

(10,15 and 20 mN/m) to form monolayer LB ﬁlms. Lifting speed was

kept at 5 mm/min. Inset of Fig. 1(A) shows the molecular structure of stearic acid

To study the effect of clay mineral Laponite in the LB ﬁlms of SO, (SA) and Safranine-O (SO). One cationic charge is associated with

À5 +

8000 ml of SO (5.0 Â 10 M) solution was mixed with the aqueous the N ion of SO. SA-SO complex molecule was formed when

Laponite dispersion (namely 80 to 10 PPM) and sonicated prior to anionic head group of SA molecule was attached electrostatically

use in the Langmuir Trough. Here cationic SO molecules interacted to the N cation of the SO molecule. SO molecules, being water

electrostatically with anionic Laponite and got adsorbed onto the soluble, lying in the aqueous subphase of the Langmuir Trough.

Laponite surface and formed clay-SO hybrid molecules in the When a Langmuir monolayer of SA was prepared at the air-water

subphase of the Langmuir Trough. After spreading of the ODA interface, then from the aqueous subphase of the Langmuir Trough

molecules at the air-water interface of the Langmuir Trough, SO SO molecules were adsorbed electrostatically on the preformed SA

tagged Laponites were further adsorbed onto the preformed monolayer at the air-water interface. With the passage of time the

cationic ODA monolayer and thus ODA-clay-SO hybrid monolayer preformed SA monolayer was replaced by the complex SA-SO

was formed at the air-water interface. After waiting for sufﬁcient monolayer at the air-water interface. It may be mentioned in this

time (1 h) to complete the reaction, the hybrid monolayer was context that chloroform solution of SA (60 ml, 0.5 mg/ml concen-

compressed slowly to obtain p-A isotherm and also transferred tration) was spread at the air-water interface and SO aqueous

À5

onto solid substrates at a desired surface pressure by Y- type solution (8000 ml, 5.0 Â 10 M concentration) was mixed in the

deposition technique to form mono- and multilayered LB ﬁlms. aqueous subphase of the Langmuir trough. The number of SA

Completion of reaction for LB ﬁlm formation was monitored by molecules present on the aqueous surface was calculated and

observing the surface pressure vs time characteristic curves (ﬁgure found to be 1.8 Â 10 and the number of SO molecules in the

not shown). After 1 h a plateau region was observed in the curve aqueous subphase of the Langmuir Trough was calculated and

indicating the completion of reaction. found to be 2.4 Â10 . Thus number of SO molecules in the aqueous

For recording the BAM images of complex/hybrid Langmuir subphase of the Langmuir Trough was more than 100 times than

monolayer, same procedure was followed to form SA-SO complex that of SA molecules on the aqueous surface. Experimentally it was

and ODA-clay-SO hybrid Langmuir monolayer. After waiting observed that the presence of sufﬁcient number of SO molecules

Fig. 2. In-situ Brewster Angle Microscopic (BAM) images of SA-SO complex Langmuir monolayer taken at different surface pressures namely (a) 5 mN/m, (b) 10 mN/m, (c)

15 mN/m and (d) 20 mN/m. Scale bar represents 20 mm.

202 M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208

À5 À4 À4

Fig. 3. [A] UV–vis absorption spectra of aqueous solution of SO at different concentrations namely 5.0 Â 10 M, 1.0 Â 10 M and 5.0 Â 10 M. Left inset shows the

À4

deconvolution spectra of aqueous solution of SO (5 Â10 M). Right inset shows the corresponding ﬂuorescence spectra of the aqueous solutions of SO at different

concentrations; [B] UV–vis absorption spectra of SA-SO complex monolayer LB ﬁlms lifted at different surface pressures namely 10, 15 and 20 mN/m along with the spectrum

À5

of the aqueous solution of SO (5.0 Â 10 M).

was important to initiate the reaction. Due to the presence of large pressure of long chain fatty acid. For pure SA monolayer the lift off

number of SO molecules each SA molecule at the air-water area was found to be 0.27 nm which was determined by the

interface interacted with one SO molecule forming SA-SO complex method described by Ras et al. [42]. At surface pressure of 15 mN/m

monolayer at the air-water interface. and 25 mN/m the areas per molecule as calculated from SA

2 2

Being water soluble, SO molecules were mixed in the aqueous isotherm, were 0.23 nm and 0.21 nm . These are in good

subphase and did not occupy any area at the air-water interface of agreement with the reported results [43]. The p-A isotherm of

the Langmuir Trough before starting the interaction. Thus at the SA À SO complex Langmuir monolayer showed a larger lift off area

beginning, area per molecule occupied at the air-water interface of of 0.61 nm . This was the clear evidence of the formation of SA–SO

the Langmuir trough was solely due to SA monolayer. With the complex Langmuir monolayer at the air-water interface. It may be

progress of the reaction SA-SO complex molecules were formed mentioned in this context that the lift off area of 0.61 nm was less

which were insoluble to water and occupied the area at the air- than the molecular area of SO under ﬂat surface conformation.

water interface. Thus pure SA monolayer was gradually replaced by Thus it became evident that SO molecules formed tilted orienta-

SA-SO complex monolayer. The area per molecule of the complex tion in the SA-SO complex monolayer at the air-water interface. At

monolayer was greater than pure SA monolayer. This was 12 mN/m surface pressure there was a phase transition which

manifested in the surface pressure vs. area per molecule (p-A) might be due to the further tilted orientation of complex molecules

isotherm characteristic study of the Langmuir monolayer at the air- at the air-water interface. This orientation might lead to a more

water interface. compact molecular organization in the monolayer. At 15 mN/m

The molecular area of SO under ﬂat surface conformation was surface pressure the area per molecule became 0.38 nm which

calculated to be about 0.78 nm . In the SA-SO complex monolayer, was greater than the area per molecule of pure SA monolayer and

long alkyl chain of SA molecule was oriented outside and SO at 25 mN/m surface pressure the area per molecule became

molecule occupied an area at the air-water interface. Thus the lift 0.28 nm . The nature of the complex monolayer isotherm was

off surface area in the isotherm curve of the SA-SO complex totally different throughout the whole surface pressure range

monolayer should be about 0.78 nm . indicating the different types of molecular organization in the

This has been discussed in the next section of the isotherm complex monolayer.

study of the complex monolayer at the air-water interface. These types of tilted organization might lead to both H- and J-

types of aggregates. From UV–vis absorption spectroscopic studies

3.2. Isotherm characteristic studies of pure SA and SA À SO complex discussed in the later section also conﬁrmed this.

Langmuir monolayer at the air-water interface

3.3. Studies of compressibility at the air-water interface

Graph (I) of Fig. 1(A) shows the p-A isotherm of pure SA

Langmuir monolayer at the air-water interface. It showed a rise of From the p-A isotherms, the compressibility data can be

surface pressure with decreasing area per molecule with a extracted. It is used to characterize the nature of monolayer phases

characteristic kink at 25 mN/m. This kink is the lateral transition and distinguish between different phases. Compressibility (C) can

M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208 203

be calculated using the following standard thermodynamic absorption band became distorted with the development of a

relation in two dimensions; high energy band with a peak at 496 nm and a longer wavelength

low energy band with peak at 534 nm.

C = À(1/A) (dA/dp)

The presence of these two bands could not be readily explained.

where A is the area per molecule at the indicated surface pressure

However it may be mentioned in this context that the aggregation

‘p’ [44,45].

of molecules modiﬁes the absorption characteristics resulting in

Phase transition of Langmuir monolayer is reﬂected by a peak in

spectral shifts and band splitting. This phenomenon can be

the compressibility versus surface pressure (CÀÀ p) curve. Fig. 1(B)

explained using the molecular exciton theory developed by Kasha

shows the (C-p) curves of pure SA monolayer (graph-I) and SA–SO

et al. [32]. Two geometrical structures are accepted in ideal case: (i)

complex monolayer (graph II) and calculated from the data of

Perfect sandwiched structure (H-dimer) in which the dipole

compression isotherms as shown in Fig. 1(A). In the 0–15 mN/m

moments of the monomeric units are aligned and in parallel planes

surface pressure region both the curves showed compressible in

with u = 90 and a = 0 , where u is the angle between the direction

nature but complex monolayer showed more compressible than

of the dipole moments of the participating chromophores and the

pure SA monolayer. In the complex Langmuir monolayer a broad

line connecting the molecular centers, a is the angle between the

band was observed in the 9 mN/m to 15 mN/m surface pressure

transition moments of the monomers in the dimer and sand-

region, indicating the more compressible nature of the complex

wiched structure in which the participating chromophores are in

monolayer in this region. The peak of the broad band at around 12

parallel planes as shown in Fig. 6(A). Such aggregation forms non-

mN/m was an indication of a phase transition of the complex

ﬂuorescent H-dimers. (ii) In-Line Head-to-Tail structure (J-aggre-

Langmuir monolayer at such surface pressure. In SA monolayer a

gate) in which the dipole moments are coplanar and in-line u = 0

small peak in the (C-p) curve at 25 mN/m surface pressure

and a= 0 and gives ﬂuorescence emission.

indicated the transition to solid phase. It is also evident from the C-

p curve that at high surface pressure region the complex Langmuir Other than the above two extreme cases, in general dye

monolayer was more compressible than pure SA monolayer. chromophores can arrange themselves with intermediate values of

u and a. Such structures present an absorption spectrum with both

3.4. In-situ Brewster Angle Microscopic (BAM) images of SA-SO H- and J-bands, called twisted structures. For structures with less

complex monolayer at the air-water interface than 54.7 , J- aggregate becomes prominent which enhances the

ﬂuorescence intensity of the chromophore, on the other hand, for u

The microstructure of the complex monolayer at the air-water greater than 54.7 H- dimer becomes prominent with diminished

ﬂ

interface can be directly visualized by in-situ BAM images. uorescence intensity.

Domains of different sizes and shapes in the BAM images of The angle can be calculated from the following relation

Langmuir monolayer indicate phase transition and formation of 2

tan (a/2) = A1/A2

micro-domains. In the present investigation in-situ BAM images of

SA-SO complex Langmuir monolayer were taken at different Where A1 and A2 are the areas of the Gaussian bands of the

surface pressures namely (a) 5 mN/m, (b) 10 mN/m, (c) 15 mN/m absorption spectrum corresponding to the longer and shorter

and 20 mN/m as shown in Fig. 2. wavelengths. In case of H-dimer, A1 is the area of the monomeric

Image (a) shows the monolayer ﬁlm containing large number of band and A2 is the area of the H- band whereas in case of J-

big circular holes having dimensions ranging from 5 mm to 15 mm. aggregates, A1 is the area of the J- band and A2 is the area of the

These dark holes represented the absence of ﬁlm and showed only monomeric band.

the aqueous surface. The white illuminated region covering the

After calculating the value of a, values of u for H- and J-

circular holes represented the complex ﬁlm. The lift off surface area

a u

2 aggregations were calculated using the equation + 2 = 180

of SA-SO complex molecule was 0.61 nm which was less than the

(From Schematic of Fig. 6(A)). In Fig. 3(A) left inset shows the

area per molecule of SO under ﬂat surface conformation. Now at

Gaussian deconvolution of UV–vis absorption spectrum of SO in

small surface pressure the molecules were away from each other À4

aqueous solution at 5.0 Â 10 M concentration. Gaussian decon-

and with increasing surface pressure they started coming closer. At

volution shows H-dimeric, monomeric and J-aggregated bands at

smaller surface area since the complex molecules were not close

positions 490 nm, 518 nm and 537 nm. From the deconvoluted

enough hence some voids were created in the monolayer ﬁlm and

spectrum the angle u = 60.5 for H- aggregation and u = 49.7 for J-

these were observed as dark circular holes in the BAM images of

aggregation were calculated. These two calculated values of u also

Fig. 2(a) and (b). With increasing surface pressure the large

satisfy the conditions for H- band (u greater than 54.7 ) and J- band

dimensional circular voids reduced to small voids as shown in the

(u less than 54.7 ).

images (b) and (c) and at 20 mN/m surface pressure, a uniform

monolayer surface was observed with the presence of very little Therefore it may be concluded that the high energy band with a

small dimensional circular voids. It indicated a uniform ﬁlm peak at 496 nm originated due to H-aggregation resulting in the

structure. Thus BAM images gave visual evidence of different formation of H-dimeric band and the longer wavelength low

phases of the complex Langmuir monolayer as observed from the energy band with peak at 534 nm was due to the formation of J-

isotherm studies. aggregates. The presence of H-dimeric band at higher concentra-

tion of SO in aqueous solution, drastically reduced the ﬂuorescence

ﬂ

3.5. UV–vis absorption and ﬂuorescence spectra of aqueous solution of intensity. Right inset of Fig. 3(A) shows the uorescence spectra

SO at different concentrations corresponding to different concentrations of SO in aqueous

solution. In all the cases the ﬂuorescence spectra were obtained

Fig. 3(a) shows the normalized UV–vis absorption spectra of SO by using the excitation wave length 490 nm. At low concentration

À5

Â ﬂ

in aqueous solution at different concentrations namely of 5.0 10 M, the solution uorescence spectrum shows intense

À5 À4 À4 À5

ﬂ

5.0 Â 10 M, 1.0 Â 10 M and 5.0 Â 10 M. At 5.0 Â 10 M con- featureless uorescence band with peak at 580 nm. Due to the

ﬂ

centration, the solution absorption spectrum showed only intense presence of non- uorescent H-dimeric species in aqueous

* À4

monomeric band with a peak at 519 nm originating due to n-p solution of SO at higher concentration (1.0 10 M,

À4

Â ﬂ

transition [1]. With increasing solution concentration the 5.0 10 M), the uorescence intensity drastically reduced.

204 M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208

3.6. UV–vis absorption spectra of SA-SO complex LB ﬁlms 501 nm where as the 519 nm monomeric band became totally

absent. Thus it became evident that at lower Laponite concentra-

Fig. 3(B) shows the normalized UV–vis absorption spectra of the tion in aqueous solution SO molecules were adsorbed on the

SA-SO complex monolayer LB ﬁlms lifted at different surface Laponite surface and formed H-dimers. While at 80 PPM

pressures namely 10, 15 and 20 mN/m along with the solution concentration in the aqueous solution adsorption of SO molecules

À5

absorption spectrum (5.0 Â 10 M). The absorption spectra onto the Laponite surface led to the formation of J- aggregation.

became broaden at all surface pressures. At higher surface pressure These two different types of organization of SO molecules onto

of 20 mN/m the UV–vis absorption spectrum showed a broaden Laponite surface can be explained on the basis of a schematic as

proﬁle with two distinguishable overlapping band having peaks at shown in Fig. 6(B).

501 nm and 529 nm. As discussed previously these two bands were It may be mentioned here in this context that cation cxchange

due to H-dimeric and J-aggregated bands. The presence of intense capacity (CEC) of Laponite is 0.74 meq/gm. 10 PPM Laponite

dimeric band at 501 nm drastically reduced the ﬂuorescence concentration in the aqueous dispersion is roughly equal to 80% of

intensity to the background level. Thus no representable ﬂuores- CEC of Laponite that is 80% of total charge on Laponite surface is

cence spectra for monolayer SA–SO complex LB ﬁlms were found. required to compensate the charges of all SO molecules present in

the aqueous subphase. Thus only 20% charge on Laponite surface

3.7. Effect of nano clay mineral Laponite on the aggregation behavior remains free. In other words we may say that Laponite

of SO in aqueous solutions concentration in aqueous subphase is quite small. Thus large

number of SO molecules got adsorbed on a single Laponite surface.

Cationic SO molecules adsorbed electrostatically on the surface While 80 PPM Laponite concentration is roughly equal to 10% CEC

of the anionic clay mineral Laponite. Laponite has a layer structure of Laponite that is 10% charge of total Laponite present in the

with a large surface charge density [46]. Concentration of the aqueous subphase is sufﬁcient to compensate all the charges of SO

Laponite affected the aggregation behavior of SO molecules in the molecules. Thus due to the availability of large number of clay

aqueous solution. mineral Laponite, fewer numbers of SO molecules got adsorbed

Fig. 4 shows the UV–vis absorption spectra of SO in aqueous onto a single Laponite surface. As shown in the schematic of

Laponite dispersion having concentrations varying from 80 to 10 Fig. 6(B), in H- aggregated pattern parallel stacking of molecules

PPM along with pure aqueous solution. At lower Laponite occurred and thus large number of molecules could be accommo-

concentration of 10 PPM, intense high energy dimeric band with dated on a single Laponite surface. Where as in J- aggregation,

peak at 501 nm was developed along with a longer wavelength molecular arrangement became head to tail types resulting in a

broad shoulder at 529 nm. With increasing Laponite concentration fewer number of molecules that could be accommodated onto a

in the aqueous solution, the 529 nm shoulder became intense and single Laponite surface. Thus availability of large number of

the dimeric peak became reduced. At 80 PPM concentration, Laponite led to the formation of J- aggregation of SO molecules

intense 529 nm peak was observed along with a weak shoulder at whereas less number of Laponite inﬂuenced the formation of H-

dimers.

Inset of Fig. 4 shows the ﬂuorescence spectra of the SO in

aqueous Laponite dispersion having concentrations varying from

À5

Fig. 4. UV–vis absorption spectra of aqueous solution of SO [5.0 Â 10 M] at

different Laponite concentrations (80–10 PPM) along with pure SO aqueous

Fig. 5. p-A isotherms of (I) ODA on pure aqueous subphase, (II) ODA-clay hybrid

solution. Inset shows the corresponding ﬂuorescence spectra of the aqueous

monolayer, (III) ODA-clay-SO hybrid monolayer.

solutions of SO at different Laponite concentrations.

M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208 205

Fig. 6. [A] Schematic representation for the condition of formation of ideal H- and J- aggregates, [B] Schematic representations of ideal H- and J- molecular aggregates on

Laponite surface [C] Schematic representation for arrangement of ODA and SO molecules on the Laponite surface and formation of H- and J- aggregates.

80 to 10 PPM. The excitation wavelength used was 490 nm. 3.8. Adsorption of SO molecules tagged clay mineral Laponite onto the

Predominance of J-aggregated sites of SO molecules in the aqueous preformed cationic octadecylamine (ODA) monolayer

solution having 80 PPM Laponite concentration, resulted in the red

shifted intense J-band in the UV–vis absorption spectrum. Since J- In order to investigate the effect of clay mineral Laponite on the

aggregated species are highly ﬂuorescent, maximum ﬂuorescence organization of SO molecules in the Langmuir monolayer and LB

intensity of the aqueous solution of the dyes occurred at this ﬁlms, cationic amphiphiles octadecylamine (ODA) was chosen to

concentration. With decreasing Laponite concentration in the prepare the template Langmuir monolayer. Being cationic ODA

aqueous solution non-ﬂuorescent H-dimeric sites became pre- molecules do not interact electrostatically with the cationic SO

dominant resulting in the quenching of ﬂuorescence intensity. molecules. For SO molecules to be adsorbed onto the ODA template

206 M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208

Fig. 7. In-situ Brewster Angle Microscopic (BAM) images of ODA-Clay-SO hybrid Langmuir monolayer taken at different surface pressures namely (a) 5 mN/m, (b) 10 mN/m,

(c) 15 mN/m and (d) 20 mN/m. The Langmuir monolayer was formed at 80 PPM Laponite concentration in the aqueous subphase of the Langmuir trough. Scale bar represents

20 mm.

monolayer, anionic clay mineral Laponite was chosen as a

mediator. In the clay Laponite dispersed aqueous subphase of

the Langmuir Trough, 8000 ml aqueous solution of SO

À5

(5.0 Â 10 M) was mixed. Being cationic, SO molecules got

adsorbed on the anionic sites of the Laponite surface. Then ODA

monolayer was prepared on the subphase containing aqueous

dispersion of SO tagged Laponite in the Langmuir Trough. Due to

electrostatic interactions, SO tagged Laponite were further

adsorbed onto the cationic ODA monolayer. As a result, ODA-

clay-SO hybrid Langmuir monolayer was formed at the air–water

interface. This is shown schematically in Fig. 6(C). As a result the

effective area per molecule surrounding one ODA molecule was

sufﬁciently increased. Therefore the area per molecule of this

hybrid monolayer was increased as evidenced from the isotherm

characteristics of the ODA-clay-SO hybrid monolayer.

Fig. 5 shows the isotherms of (I) pure ODA monolayer, (II) ODA-

clay-hybrid monolayer and (III) ODA-clay-SO hybrid monolayer.

ODA isotherm on pure aqueous subphase shows a steep rising

indicating the highly condensed and low compressible character-

istics of ODA monolayer and same as reported elsewhere [47].

ODA-clay hybrid monolayer isotherm was measured on the

Laponite dispersed aqueous subphase at ambient condition with

freshly prepared deionized distilled water. From the ﬁgure it was

observed that ODA-clay hybrid monolayer isotherm has higher

area per molecule than that of pure ODA isotherm which was the

clear evidence of the adsorption of clay mineral Laponite onto the

cationic template ODA monolayer. From the isotherm of ODA-clay-

SO hybrid monolayer it was observed that the area per molecule of

ODA-clay-SO hybrid monolayer was higher than that of even ODA-

clay hybrid monolayer.

Fig. 8. UV–vis absorption spectra of ODA-Clay-SO hybrid monolayer LB ﬁlms lifted

Laponite has a layer structure having a large number of anionic

at different Laponite concentrations (80–10 PPM). All the LB ﬁlms were lifted at a

sites on the surface. When it was adsorbed from the aqueous ﬁxed surface pressure of 10 mN/m. Inset shows the corresponding ﬂuorescence

spectra of the same hybrid LB ﬁlms.

M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208 207

dispersion of the Langmuir Trough onto the template cationic ODA 20 mN/m these domain structures almost disappeared and BAM

monolayer at the air–water interface, cationic head group of the images showed almost uniform spread of white illuminated

ODA molecules interacted electrostatically with the anionic sites of smaller domains. Thus the ﬁlms were spread uniformly over the

the Laponite surface and thus the ODA molecules were tagged on whole region. From the BAM images recorded before and after

the Laponite surface. This was schematically shown in Fig. 6(C). inclusion of Laponite, it became evident that inclusion of Laponite

However it is not possible to conﬁrm quantitatively the exact affected the molecular organizations in the Langmuir monolayer at

number of ODA molecules adsorbed on the Laponite surface since the air-water interface. Clay mineral Laponite has dimensions

there were large numbers of anionic sites present on the Laponite varying from 50 nm to 100 nm. However the domains observed in

surface. But since upon adsorption on the Laponite surface, the the BAM images have several micrometer dimensions. Therefore it

effective area surrounding one ODA molecules was increased, it became evident that several ODA-clay-SO hybrid molecules

resulted in the overall increase of area per molecule of the ODA- aggregated to form microcrystalline domains. The nature of the

clay hybrid and ODA-clay-SO hybrid monolayer. aggregated species could not be ascertained from the BAM images

however UV–vis absorption and Fluorescence spectroscopic

3.9. In-situ Brewster Angle Microscopic images of ODA-clay-SO hybrid studies discussed in the next section clearly showed the nature

monolayer at the air-water interface of the aggregated species.

BAM images of the ODA-clay-SO hybrid Langmuir monolayer 3.10. Effect of clay mineral Laponite on the changes in molecular

were taken at different surface pressures of 80 PPM Laponite aggregates of SO in LB ﬁlms

concentration in the aqueous subphase. Fig. 7 shows the BAM

images at (a) 5 mN/m, (b) 10 mN/m, (c) 15 mN/m and (d) 20 mN/m Fig. 8 shows the normalized UV–vis absorption spectra of the

surface pressures of the hybrid Langmuir monolayer at the air ODA-clay-SO hybrid monolayer LB ﬁlms prepared from the

water interface. Langmuir monolayer on the subphase of the Langmuir Trough

In the BAM images of ODA-clay-SO hybrid Langmuir monolayer, having Laponite concentrations varying from 80 to 10 PPM. Surface

distinct hexagonal shaped domains were observed at 5 mN/m pressure of lifting was kept ﬁxed at 10 mN/m. UV–vis absorption

surface pressure. These domains were larger in size. It may be spectrum of monolayer LB ﬁlms prepared at 10 PPM Laponite

mentioned in the context that in the BAM images white concentration showed a broad higher energy H-dimeric band with

illuminated region indicated the presence of materials and the peak at 503 nm along with a weak lower energy shoulder of J-band

black region is the absence of materials. Thus in the hexagonal at 531 nm. With increasing Laponite concentration, 531 nm J-band

domains the black bordering indicated the absence of materials. became intense. At 80 PPM Laponite concentration the broad H-

However with increasing surface pressure these domains became dimeric band at 503 nm in the hybrid monolayer LB ﬁlm became

smaller in dimension. At higher surface pressures of 15 mN/m and totally absent and the longer wavelength J-band became intense.

Fig. 9. [A] UV–vis absorption spectra of different layered hybrid LB ﬁlms at 80 PPM Laponite concentration. Inset shows the corresponding ﬂuorescence spectra of the same

layered hybrid LB ﬁlms; [B] UV–vis absorption spectra of different layered hybrid LB ﬁlms at 10 PPM Laponite concentration.

208 M. Saha et al. / Journal of Photochemistry and Photobiology A: Chemistry 348 (2017) 199–208

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